1. Field of the Invention
The present invention relates to a sampling method in a photon correlation spectroscopy used for, for example, a particle size analyzer or the like.
2. Description of the Background Art
In measurements of particle size distribution based on the photon correlation spectroscopy (PCS), in addition to counting the number of photons (pulse signal) at a given sampling time depending on a scattered light intensity arising from particles to be measured, time is delayed with a shift register; a product-sum operation is performed to obtain an autocorrelation function; and a particle size (particle diameter) is calculated from a relaxation coefficient of the function.
Also, as the sampling method, a linear sampling method, exponential sampling method, multi-tau method, and the like are known.
However, these methods respectively have the following problems:
The autocorrelation function obtained by the photon correlation method is typically an exponential function for a particle with only monodispersive, and a shorter sampling time leads to a calculation result with higher accuracy; however, the linear sampling method is one in which the autocorrelation function of a pulse train is calculated at equal time intervals, and simple in circuit configuration and a control signal, but requires a large number of channels if a wide range of particle sizes is calculated with high accuracy. Also, even if the number of channels is sufficient for calculation, the sampling time should be adjusted depending on a particle size, resulting in the trade-off between the number of channels and an accuracy or time-consumption, and therefore the linear sampling method has a problem of practical difficulty in the use for the wide range of particle sizes.
Also, in the exponential sampling method, the sampling time is different for each channel so as to be able to cover the shortcoming of the above linear sampling method and be applied to the wide range of particle sizes, and as a stage proceeds, set to exponentially increase depending on a channel. However, in practice, the method has a problem that increasing the sampling time leads to a shortage of the number of data points on small particles, resulting in a reduction in accuracy, or the like.
Further, the multi-tau method is, as described in Patent document 1, one in which all channels are divided into blocks, and by linear sampling for channels in each of the blocks and exponential sampling between the blocks, the shortcomings of the above both methods are overcome and the advantages of the both are adopted, but has a problem that for a channel having a longer interval in a later stage block, a base line fluctuation, i.e., so-called bias noise, is increased, resulting in a reduction in accuracy of the autocorrelation function.
Patent document 1: Japanese Unexamined Patent Publication No. 2002-296118